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NOAA TR NESS 62 



A UNITED STATES 
DEPARTMENT OF 
COMMERCE 
PUBLICATION 



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35 



NOAA Technical Report NESS 62 



U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Environmental Satellite Service 



WASHINGTON, D.C. 
October 1972 



Proposed Calibration Target 
For the Visible Channel of 
a Satellite Radiometer 



K. L. COULSON 
H. JACOBOWITZ 




NOAA TECHNICAL REPORTS 

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(Continued inside back cover) 



^ 0f c 0a 




^ATES O* h 



U.S. DEPARTMENT OF COMMERCE 
Peter G. Peterson, Secretary 

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION 
Robert M. White, Administrator 

NATIONAL ENVIRONMENTAL SATELLITE SERVICE 
David S. Johnson, Director 



NOAA Technical Report NESS 62 

Proposed Calibration Target for the 
Visible Channel of a Satellite Radiometer 



K. L. COULSON 
H. JACOBOWITZ 






o 

n 

o 

< 



WASHINGTON, D.C 
OCTOBER 1972 



UDC 551.507.362.2:551.508.21 

551.5 Meteorology 

.507 Instrument carriers 

.362.2 Satellites 

.508 Meteorological instruments 

.21 Radiometers 



For sale by the Superintendent of Documents, U.S. Government Printing 
Office, Washington, D.C. 20402. Price 35 cents. 



CONTENTS 

Page 
Introduction 1 

Theoretical considerations . . 

Case of a Rayleigh atmosphere 3 

Case of a turbid atmosphere with ozone 12 

Required Measurements 18 

Measurements of the direct flux F D at the surface 18 

Measurements of global flux 21 

Measurements of surface reflectance 21 

Suggested instrumentation and observations 23 

Instrumentation for routine operations 23 

Instrumentation for special measurements 24 

Auxiliary observations 25 

Discussion 25 

References 27 

LIST OF FIGURES 

Figure 1. — Total optical thickness and optical thickness of the 
various components of the Elterman 1968 model of a turbid 
atmosphere at White Sands 5 

Figure 2. — Relative flux F (-y ) at White Sands at various solar 

zenith angles for a Rayleigh atmosphere 5 

Figure 3. --Relative flux F^ downward at the surface of White 

Sands for a Rayleigh atmosphere and various solar zenith angles. . . 6 

T 

Figure 4.— Relative flux F^ s + F d + F _ downward at the surface 
of White Sands for a Rayleigh atmosDhere and various solar 
zenith angles 7 

Figure 5. — Spectral reflectance of a sample of white gypsum sand 

from New Mexico, as measured by Hovis (1971) • 8 

iii 



Page 

Figure 6. --Total relative flux downward at the surface of White 
Sands for a Rayleigh atmosphere and various solar zenith angles. 
Arrows indicate relative flux incident at the top of the 
atmosphere 9 

Figure 7. --Total spectral flux of radiation incident at the surface 
of White Sands for a Rayleigh atmosphere and various zenith 
angles of the sun 10 

Figure 8. — Intensity I - and I . + I c of solar radiation directed 

gD gd S 

outward from the nadir direction at the top of a Rayleigh 

atmosphere over White Sands 11 

Figure 9. — Total intensity of solar radiation directed outward 

from the top of a Rayleigh atmosphere over White Sands 12 

Figure 10. — Mean of 79 vertical profiles of the aerosol attenuation 
coefficient, compared to that of a Rayleigh model, for the 
atmosphere over White Sands (after Elterman 1968) 14 

Figure 11. — Fractional transmission of Elterman 1968 model of a 
clear atmosphere over White Sands as a function of wavelength 
for various zenith angles of the sun. A curve for the Rayleigh 
model at =0° is given for comparison 15 

Figure 12. — Relative flux of the various components of solar 

radiation downward at White Sands for an aerosol atmosphere and 
various solar zenith angles 16 

Figure 13. — Total relative flux of solar radiation downward at 

White Sands for various solar zenith angles 17 

Figure 14. --Components of the relative intensity outward from the 
nadir direction at the top of a turbid atmosphere over White Sands 
for a solar zenith angle =0° ig 

Figure 15. — Components of the relative intensity outward from the 
nadir direction at the top of a turbid atmosphere over White Sands 
for a solar zenith angle 0°= 60° 19 

Figure 16. — Total absolute intensity outward from the nadir 
direction at the top of a turbid atmosphere over White Sands. 
(Incident flux as given by Thekaekara) 20 

Figure 17. — Bidirectional reflectance of white gypsum sand as a 
function of nadir angle in the principal plane for two different 
zenith angles of the source at a wavelength of 0.643 urn (after 
Coulson 1965) 22 

iv 



PROPOSED CALIBRATION TARGET FOR THE VISIBLE CHANNEL OF A 

SATELLITE RADIOMETER 

K. L. Coulson and H. Jacobowitz 

National Environmental Satellite Service 
National Oceanic and Atmospheric Administration 
Washington, D. C. 



ABSTRACT. A method is proposed for calibrating the 
visible channel of a satellite radiometer from orbit 
by using a sunlit area on the earth's surface as a 
calibration target. For a highly reflective surface 
and solar elevations of 30° or greater, the dominant 
component of the intensity of radiation directed out- 
ward from the top of the atmosphere is attributable 
to incident solar radiation which is transmitted 
directly downward through the atmosphere, reflected 
from the surface, and transmitted directly back out 
through the atmosphere. Aside from the solar constant, 
the only parameters that must be known to determine this 
dominant intensity component are the directional 
reflectance of the surface and the optical thickness of 
the atmosphere. Both can be measured directly with the 
proposed instrumentation. The intensity components 
arising from diffuse transmission or backscatter can be 
determined by measuring the global flux incident at the 
surface and applying radiative transfer theory for 
realistic models of the turbid atmosphere over the 
calibration site. A single filter instrument for the 
measurement of the global flux is suggested. A prelim- 
inary survey indicates that the white gypsum sand of 
the White Sands National Monument, N. Mex. , may be the 
most suitable calibration target within the United 
States. If a suitable surface observation station could 
be established, another very attractive possibility is 
the Solar de Uyuni, a large salt flat at an altitude 
of 12,000 feet in Bolivia. 



1. INTRODUCTION 

Because there is no reliable on-board calibration for the visible channel 
of the ITOS scanning radiometer, there is a need for some method of calibration 
that can detect instrument changes following prelaunch calibration. One 
serious problem is the drift of instrument response with time, as observed 
with earlier radiometers. Another is the possibility that instrument proper- 
ties are modified during the launch itself. Unless these changes are detected 
and appropriate corrections are applied, the data acquired in operations can 



_\ 



become unreliable and misleading. 

Perhaps the simplest way to calibrate the instrument while it is in orbit 
is to use an area on the earth's surface as a calibration standard. Such a 
"standard surface", can be viewed repeatedly by the radiometer as the 
satellite passes over the selected area. By knowing the reflectance of the 
standard surface and the characteristics of the intervening atmosphere, it 
is possible, in principle, to compute the intensity of radiation directed to 
space from the standard surface to the satellite; thus the standard surface 
becomes a known calibration source from which instrument response can be 
determined. In practice, the problem is not so simple, mainly because of 
unknown atmospheric characteristics. However, by the method described below 
it should be possible to minimize these uncertainties to yield a reliable and 
reasonably accurate calibration source for the visible (and perhaps the near 
infrared) channel of the radiometer. 

2. THEORETICAL CONSIDERATIONS 

According to Coulson et al. (1965), the solar radiation returning to space 
from the top of the atmosphere consists of five separate components, each 
component having had its own transmission-reflection history in the atmosphere- 
surface system. The five components are described below. In the symbolic 
notation the subscript D indicates direct transmission and d indicates diffuse 
transmission; the first subscript is for the incoming radiation, the second 
for outgoing radiation. 

Component No. 1: Ij^ + y , <J) ) - Radiation transmitted directly in its down- 
ward traverse of the atmosphere, reflected from the surface, and transmitted 
directly back out through the atmosphere. In general, the intensity is a 
vector quantity, but for an analysis of the energetics of the system, polar- 
ization effects can be reasonably neglected. In that case, I-q-q is the scalar 
quantity (Coulson et al. 1965) 

I ( + y,<f>) =^F y p(y,c(>;y ,(J) )e-L^o) (!) 

Here, y and y are the cosines of the zenith angle 9 of the sun and the nadir 
angle 6 of the upward radiation, respectively, <J> and 4> are the corresponding 
azimuth angles, tt F q is assumed to be the flux of solar radiation normal to 
the beam at the top of the atmosphere, p is the bi-directional reflectance of 
the surface, and x is the normal optical thickness of the atmosphere. 

Component No. 2: I Dd (+y yf> ) - Radiation transmitted directly through the 
atmosphere, reflected from the surface, and transmitted diffusely back out 
through the atmosphere. It is given in scalar form by the expression 

2 1 

I Dd (+y,<j>)= ^o^o e " T/M ° JJ p (vA;V A Q ) T(y 3 (}>;y,cj>) dy d<j> , ( 2 ) 

f o 

where T ( y , <j> jp <j>j is the transmission function of the atmosphere 
(Chandrasekhar 19 50). 

Component No. 3: I dD (+ y , <t> ) - Radiation transmitted diffusely through the 
atmosphere, reflected from the surface, and transmitted directly back out 



3. 

through the atmosphere. Methods of approximating this and the following 
components are discussed later. 

Component No. k' ^dd^ + ^ " Radiation transmitted diffusely through the 
atmsophere, reflected from the surface, and transmitted diffusely back out 
through the atmosphere. 

Component No. £: I s (+ y, <f>) - Radiation is backscattered by the atmosDhere 
without ever having reached the surface. It can be exoressed in the form 

I s (+M) = I S ( y*^ o»*o )7rF o ' (3) 

4-TTy 

where S( y , <J> ;y <j> ) is the reflection function of the atmosphere 

(Chandrasekhar 19§0) . 

It is tempting to try to compute each of these components individually and 
thereby derive an intensity against which the radiometer could be calibrated. 
Cur knowledge of atmospheric properties is still too meager, however, to 
obtain intensity values of sufficient accuracy and reliability for calibration 
purposes from theory alone. Thus it is proposed below that the theoretical 
calculations be bolstered by surface measurements to yeild reliable and 
sufficiently accurate values of the outward intensity for radiometer 
calibration. 

In general, the optical thickness t, the transmission function T, and the 
reflection function S of the atmosphere are all dependent on the number, type, 
and spatial distribution of aerosols in the atmosphere. For a quantitative 
determination of the radiation available for calibrating the orbiting radio- 
meter, the aerosol effects must be taken into account. This would present no 
great problem if the characteristics of the aerosols were known at the time 
of observation. Unfortunately, such knowledge is not generally available, so 
recourse to calculations based on aerosol models is necessary. Results for 
selected aerosol models are outlined in a subsequent section. 

A. Case of a Rayleigh Atmosphere 

As a preliminary step, consider the case of a Rayleigh atmosphere over- 
lying a surface of white gypsum sand. Such a model should be a reasonably 
good approximation in the present problem for a number of reasons. First, 
the white gypsum sand exhibits a reflection pattern which is not strongly at 
variance with the Lambert case. Secondly, the aerosol content of the atmos- 
phere of New Mexico is normally very low. Thus the Rayleigh approximation 
should not be too far from reality at White Sands, particularly for computa- 
tions of the flux due to skylight. Finally, the skylight component is 
diffuse in character, thus minimizing directional effects at the surface, it 
constitutes only a small part of the total energy incident at the surface, 
and the effects of aerosols on the skylight would be only a small perturbation 
quantitatively. 

The scheme is to find the total monochromatic global flux of radiation 
F e (- y) incident at the ground, reflect it from the white sand to get the 
outward intensity I g (+y ) at the ground, transmit this intensity directly 



u. 

and diffusely to the top of the atmosphere to obtain Ig-D^" 1 " y) anc * ^gd^* v) s 
and finally add the intensity I s ( + y) due to backscattering of the original 
incident flux. Thus the total intensity from the nadir direction at the top 

of the atmosphere is 

I(+y) = I gD (+y) + I gd (+y) + I g (+y) (^ 

As discussed by Goulson (l°68) , the radiative flux F _(-y ) incident on the 
ground consists' of the following five components: 

F-p>(- y Q ) : radiation transmitted directly through the atmosphere as the 
original parallel beam from the sun. 

F Dg (- y) : the part of F D (- U q , <J> ) reflected from the surface and back- 
scattered by the overlying atmosphere in the first reflection- 
backscatter cycle. 

F, (-y ) : radiation transmitted diffusely on its traverse of the 
atmosphere. 

F ds (-y ) * the part of F,(-y ) reflected from the surface and backscattered 
by the overlying atmosphere. This includes all orders of reflection- 
backscatter cycles of component F.(-y ). 

Ftj '(- y) : a small flux due to all reflect! on-backscatter cycles of F^(-y ) 
of order higher than the first. 

Suscripts D and d indicate, respectively, the direct and diffuse transmission 
of the original incident radiation, and s indicates the scattering of the 
reflected radiation. 

For the wavelengths of interest here (0.£y m to 0.9y m) , Fp(-p Q ) is by far 
the largest. It is given by the simple exponential relation 

F D^ o> - * V o e ~ T/V " (5) 

where tt F Q y is the monochromatic flux incident on a horizontal surface at 
the top of the atmosphere. Figure 1 shows the dependence of optical thickness 
on wavelength for the aerosol, Rayleigh, and ozone components of the atmos- 
phere for the elevation of White Sands, interpolated from the tabulations of 
Elterman (l°68) . Computations of relative values of F^(-y Q ) using the 
optical thickness of the Rayleigh atmosphere and the relation of (5), were 
made at intervals of 0.01 y m from A. =0.5 to 0.9V 1 m for seven different 
zenith angles of the sun. The curves of figure 2 show the results. For 
these computations, F Q has been set to unity. The fractional transmission 
of the Rayleigh atmosphere varied from 0.882 for X =0.5 to 0.988 for 
A =0.9 ym for the sun at the zenith. For a solar zenith angle of 60°, 
the corresponding values are 0.777 and 0.977. 

Curves for component F^(-y ), obtained by a double interpolation from the 
tabulation by Coulson (1968), are shown by figure 3- This component shows 
surprisingly little dependence on zenith angle of the sun, and it is quite 
small compared to the directly transmitted flux. In fact, for the sun at 




Figure 1. Total optical thickness and optical thickness of the various 
components of the ELterman 1968 model of a turbid atmosphere at 
White Sands, 




0.5 0.6 0.7 0.8 

Wavelength (um) 



Figure 2. Relative flux F {- v Q ) at White San^s at various solar zenith 
angles for a Rayleigh atmosphere. 



6. 



0.25 



0.20- 



X 



4) 
> 

a> 



0.15- 



0.10- 



0.05- 



0.00 




Wavelength (jmm) 



Figure 3. --Relative flux F^ downward at the surface of Miite Sands for 
a Rayleigh atmosphere and various solar zenith angles. 

the zenith F D (- y ) is only 6.8 percent of Fj^(-p Q ) at A =0.5 m and 0.6 
percent of Fj-)(-y ) at A =0.9 m. At o =60", "the values increased to lh«3 
percent and 1.2 percent, respectively. 

The three components F^ s F , , and Fj-j s all arise because of the relection 
and backscatter of the light incident on the surface. Their sum has been 
tabulated for selected parameters by Coulson (1968), from which the curves 
of figure h were obtained by multiple interpolation. The reflectance of the 
white gypsum sand (fig. 5) was taken as that measured by Hovis (1971). The 

.(-V ) is of the same order as that of 
:hat backscatter of the surface -reflected 
radiation by the atmosphere results in the average intensity of skylight 
being approximately doubled because of reflection from the white sand surface, 



magnitude of the sum F^ (-y ) + FjJ 
F H (-y ) alone, a fact which means tl 



Another interesting feature (shown in fig. 6) is that the flux incident 
at the surface is almost the same as that assumed incident at the top of the 
atmosphere. Physcially, this means that the amount of energy lost to the 



0.25 



0.20- 



X 



> 

ex. 




0.05- 



0.00 



Wavelength (jum] 



Figure 4. — Relative flux F^ s + F^ s + Fjj g downward at the surface of White 
Sands for a Rayleigh atmosphere and various solar zenith angles. 

system through being scattered back to space by the atmosphere is compensated 
for by part of the energy being directed back to the surface after single or 
multiple reflection by the white gypsum sand. (The atmosphere is assumed to 
be nonabsorbing. ) Only for such a highly reflecting surface would so nearly 
complete compensation occur. 



Changing the relative fluxes, shown so far, to abs 
only a multiplication of the relative values by F /tt 
absolute energy in the solar beam at the top of the 
distribution in absolute energy units of the total g 

+ ^Ds (~^ ) + ^ds( _1J ) + Fds( - ^ ) incident at the surface at 
for the case of a Rayleigh atmosphere in figure 7. 
combined with energy backscattered by the atmosphere 
the top of the atmosphere to yield the intensity fie 
for calibrating satellite radiometers in orbit. The 
what the intensity of this outward radiation will be 



olute fluxes requires 

(where F is the 
atmosphere). The spectral 
lobal flux F g (-y) =F D (-v) 

White Sands is shown 
Part of this energy 

itself is returned to 
Id which might be used 

next task is to determine 



It is convenient to consider the outward radiation as consisting of the 




E 



D) 

c 
Q) 

> 
O 





CO 
CO 



o 
o 



!3 




CO 



CO 

ft 



p 

•H 

cm 
O 



I 

CO 



O 


O 

cti 
-P 
o 
Q) 





oj • 

-P rH 
O P- 

On 



fiv 



(4uaDjaj) aDUD4Da|jay 



I CO 

• -H 

"LA !> 

O 

W 

•H 




0.6 0.7 0.8 0.9 

Wavelength (yum) 



Figure 6. --Total relative flux downward at the surface of White Sands 
for a Rayleigh atmosphere and various solar zenith angles. Arrows 
indicate relative flux incident at the top of the atmosphere. 

following three intensity components: 



L gD 



(+p) 



hd (+ m) 



Io(+y) 



surface -reflected radiation transmitted directly through 
the atmosphere. 

surface-reflected radiation transmitted diffusely outward 
through the atmosphere. 

radiation backscattered by the atmosphere without having 
ever reached the surface. 



We again assume, as a first approximation, a Rayleigh atmospheric model and 
further assume that the radiometer is viewing directly downward over the 
White Sands area. This latter assumption is by no means necessary, but is 
realistic for an orbiting satellite. Generalization to non-nadir directions 
will present no difficulty, if such is necessary.' 

The first requirement is to determine the intensity I g ( + I- 1 ) in the outward 
direction at the ground. For the Lambert surface assumed, this intensity is 
independent of direction and is of magnitude 



lj + v) - 



R 



V-io , 



(6) 



where R is total reflectance of the surface. Values of R and F (- u) are 



10. 




0.6 0.7 0.8 

Wavelength (urn) 



09 



Fig. 7. --Total spectral flux of radiation incident at the surface of White 
Sands for a Rayleigh atmosphere and various zenith angles of the sun. 

given in figures 5 and 7, respectively. 



By far the most dominant component of the outward intensity for the case of 
White Sands is I„n( + ). For the nadir direction it is expressed as 



i gD ( + 



IgD^u) 



y„). 



- T 



(7) 



Curves of 1~q(+ \i) are shown as functions of wavelength for seven different 
solar zenith angles in the top section of figure 8. It should be noted that 
the units (W cm~2 ym~l sr"- 1 -) are those of intensity, not of flux. 

Values of component Ig(+y) can be obtained by triple interpolation from 
the tables of Coulson, Dave, and Sekera (i960) using the appropriate values 
of surface reflectance for White Sands and the variation of optical thickness 
with wavelength at the altitude of White Sands. The accuracy obtained by 
this procedure Is not high, but Ig(+y) Is a small quantity compared to 
I g £)( + y). The interpolation errors are of second order and probably 
negligible. 



No equivalent set of calculations is available for determining directly the 
component I ^(+y), but It too is a small quantity at the wavelengths of 
interest, so any errors resulting from its approximation will be of minor 
significance. The calculations necessary to determine I , (+y) precisely for 



11, 



0.040 



0.035 














10 




0.030 


20 

30 


5; 


0.025 


40 


i 
E 


0.020 


50 


£ 






2- 




60 



0.015 



0.010 



0.005 



0000 




0,10 




05 



06 0.7 0! 

Wavelength (urn) 



-id 
09 



Figure 8. — Intensity 1„-q an( ^ I 



, + I„ of solar radiation directed outward 
iRd s 

from the nadir direction at the top of a Rayleigh atmosphere over White 
Sands. 

a given atmospheric model are not difficult, but a suitable program is not 
immediately available. To use the tabulation of Coulson, Dave, and Sekera 
(i960) for the purpose, it was assumed that, because of diffuse transmission 
of relfected light, the emergent intensity from the nadir direction at the 
top of the atmosphere would be the same as the skylight intensity from the 
zenith direction at the bottom of the atmosphere for the same energy input. 
This should be completely valid by the reciprocity theorem (transfer character- 
istics invariant to exchange of incident and emergent directions) if the 
angular distribution of the two incident streams of radiation were the same. 
Such is not the case here; the sunlight incident at the top of the atmosphere 
is assumed to be monodirectional while that actually entering the base of the 
atmosphere after reflection from the surface is nearly isotropic. The worst 
possible error that could be introduced by this approximation is a factor of 
two, which would apply for a single scattering particle on which the two 
streams of incident radiation are normal to each other. The least possible 
error would be zero, which could occur at some specific intermediate angles 
of incidence of the monodirectional beam. The actual error from the approxi- 
mation is between these two extremes. In view of the small inherent intensi- 
ties involved, this error is probably negligible in comparison to those 
arising from the other assumption used. 



The sum of the two components Ig( + y ) and Ip-d^ + y ) ^ s plotted as a function 



12. 



0.040 




0000 



5 6 7 8 9 

Wavelength (itm) 



Figure 9. --Total intensity of solar radiation directed outward from the 
top of a Rayleigh atmosphere over White Sands. 

of wavelength in the lowe'r section of figure 8. A small extrapolation was 
necessary to extend the data of Coulson, Dave, and Sekera (i960) to wave- 
lengths beyond X=0.8y m. Figure 9 shows the total intensity of radiation 
emerging from the nadir direction for the model of Rayleigh atmosphere and 
Lambert surface over the White Sands area. This represents the maximum 
intensity that would be available for the calibration of orbiting satellite 
radiometers. Aerosol effects and the absorption by atmospheric ozone will 
tend to decrease the outward intensity from that obtained for the Rayleigh 
case. 

B. Case of a Turbid Atmosphere With Ozone 

The real atmosphere over White Sands always contains an aerosol component 
(dust, haze, smoke, and other types of particles) and some ozone. Character- 
izing these additional components is a serious problem in making a theoretical 
analysis of the radiative transfer that might exist at a given time at White 
Sands. The total amount of ozone is not known and, although the (Chappuis) 
absorption bands for ozone are relatively weak, they are broad and their 
cumulative effect is by no means negligible. Not only is there uncertainty 
about the aerosol content of the atmosphere, but optical properties, such as 
size -frequency distribution and index of refraction of the aerosols, are 
likewise still largely unknown. Thus, from a computation standpoint we are 
restricted to the use of models for determining logically what might occur. 
Even though models may reveal little about what actually occurs in a specific 



13. 

case, the results of modeling are useful in setting limits on what might 
reasonably be expected. 

A large number of atmospheric models are available from which to choose. 
The model chosen here is that of Elterman (1968). The main consideration for 
this choice is that it is based on 79 aerosol profiles obtained in the 
immediate area of White Sands by searchlight techniques. This model should 
represent atmospheric optical conditions over White Sands as validly as is 
feasible at the present time. In addition, the large number of measurements 
should permit the setting of some realistic limits on the variations to be 
expected. 

The mean of 79 vertical profiles of the attenuation coefficient for the 
atmosphere over White Sands for^ =0.55>ij m is shown in figure 10. The data 
were extrapolated from the level of the searchlight receiver (2.76 km) to 
sea level by assuming a scale height (height range required for a change in 
the coefficient by a factor of 1/e) of 1.2 km. Since the surface of the white 
gypsum sand is at an altitude of 1.22 km, the largest attenuation coefficients 
near sea level are avoided. Still, aerosol attenuation in the model is consid- 
erably greater near the surface at White Sands than it would be for the Ray- 
leigh atmosphere. The attenuation is comparable to the Rayleigh atmosphere 
throughout most of the middle and upper troposphere and stratosphere, and 
considerably lower in the region of the mesosphere. 

The total ozone content of the atmospheric model of Elterman was set at 
0.35 cm at standard temperature and pressure, a typical value for a mid- 
latitude station. However, the ozone content is quite variable over short 
time periods and seasonally. For accurate calculations for the satellite 
radiometer it would be desirable to use values measured at the time. Accord- 
ing to D. Heath of NASA (private communication) total ozone content can be 
obtained to +5 percent accuracy from ultraviolet backscatter measurements from 
satellites, and from ozone measurements made routinely with a Dobson spectro- 
meter in Colorado. Either source would be adequate for the purpose. If, 
however, as is suggested below, atmospheric attenuation is monitored 
continuously at White Sands for satellite calibration purposes, a knowledge 
of ozone content would not be necessary. Its effect would be taken into 
account implicitly in the surface measurements. 

Turning now to a determination of the energy available for radiometer cali- 
bration in the case of the real turbid atmosphere over White Sands, we again 
discuss the various components of the radiation field at the surface and at 
the top of the atmosphere. It is well to remember for this purpose that the 
energy removed from a stream of radiation by attenuation is not necessarily 
lost to the radiation field. Attenuation is produced by both absorption and 
scattering. Absorption results in loss of radiant energy by conversion to 
some other form of energy; scattering simply produces a change of the direction 
of propagation of the energy. In the present problem, a considerable part of 
the energy from the original solar beam is scattered downward and still serves 
to illuminate the white sand at the surface. Likewise, part of the energy 
from the radiation directed outward is scattered upward and so can reach the 
radiometer. 

The fractional transmission of the atmosphere is given by the exponential 



lU. 



50 


— i i i imi| — i i i rnnr I i i "mi — n 

\ \ 
\ \ 


mini — i i i iini| — i i iiiiii 




\ \ 


■ 




\ \ ^MOLECULAR 




\ \^ 






\ \ 




40 


\ \ 
\ \ 
\ \ 
\ \ 






\ \ 






\ \ 






\ \ 






\ \ 




30 


\\ 






\\ 








V- AEROSOL 


20 




\\^-'1968 






V 



' I ' """I i i I mill 

I0" 7 I0" 6 10" 



ujj 



10" 



10 



ATTENUATION COEFFICIENT (km ) 



Figure 10. --Mean of 79 vertical profiles of the aerosol attenuation 
coefficient, compared to that of a Rayleigh model, for the atmosphere 
over White Sands (after Elterman 1968). 

T = e~ ' ' for the optical thicknesses of the Elterman model, which includes 
all three components (aerosols, ozone, and Rayleigh particles). This is shown 
for various values of the solar zenith angle in figure 11. A curve for the 
Rayleigh atmosphere alone (at O = 0°) is included for comparison. Although 
the shapes of the curves are qualitatively similar, the curves for the turbid 
atmosphere show a transmission of roughly 10 percent less than that for the 
Rayleigh atmosphere. The dip in the curves in the 0.5>8 to 0.65 Pm region is 
due to absorption by ozone. 

The flux of direct solar radiation Fjj(- y Q ) incident on a horizontal surface 
at the top of the atmosphere considering only the aerosol component is 
obtained by applying the equations of transfer to the aerosol component only. 
The results of computations for various solar zenith angles are shown in the 
top part of figure 12. 

Determination of the diffuse radiation components is not so simple in the 
case of the turbid atmosphere. The problem of multiple scattering between 
aerosol particles and Rayleigh particles, and among the aerosol particles 
themselves, has never been solved in a completely satisfactory manner. In 
addition, a large expenditure of computer time is required for model calcula- 
tions because of the complexities inherent in the Mie scattering theory. 

The computation problem is somewhat alleviated if we neglect multiple 
scattering of radiation between aerosol particles and gaseous molecules, but 
still account for that among the gaseous molecules and aerosol particles. 



15. 



01 

u 

c 
o 



c 
a 

o 
E 




0.5 



0.6 0.7 0.8 
Wavelength [jum] 



Figure H.- -Fractional transmission of ELterman 1968 model of a clear 
atmosphere over White Sands as a function of wavelength for various 
zenith angles of the sun. A curve for the Rayleigh model at O = 0° 
is given for comparison. 

This is a reasonable approximation for the atmosphere over White Sands, since 
the aerosol content of the atmosphere is quite low in that region. However, 
before the method is used for actual satellite calibration it would be well 
to account for complete multiple scattering of the radiation on both its 
downward and upward traverses of the atmosphere. It is possible to determine 
the total optical thickness of the atmosphere from measurements of the flux 
in the direct solar beam received at the surface, and from that to determine 
a reasonably accurate value of the total particle concentration for a 
representative size frequency distribution. This will permit the choice of 
one of a series of atmospheric models for which radiative computations need 
be performed only once. 

For the present problem, we have chosen to compute the radiative fields for 
the aerosol atmosphere over White Sands by the matrix method of Twomey, 
Jacobowitz, and Howell (1966) . This method is based on the doubling technique 
for multiple scattering calculations. By this method the radiative fields for 
an atmosphere of any desired optical thickness can be built up by computing 
the fields for a very thin layer for which single scattering calculations are 
a good approximation. The technique is repeated until the required optical 



16. 







6 .7 .8 .9 

Wavelength (yum) 



LF 9shM> 



Figure 12. --Relative flux of the various components of solar radiation 
downward at "White Sands for an aerosol atmosphere and various solar 
zenith angles. 

thickness is achieved. The resulting field includes the effects of all 
orders of multiple scattering by the scattering centers of the medium. The 
size -frequency distribution of the aerosols was taken here to be the Junge 
distribution, with the exponent set at y = -1;.0 ( Junge, 1952). 

In principle, it should be possible to use the doubling technique to 
account for multiple scattering among all the particles of the medium, both 
aerosols and Rayleigh particles. The resulting field could be determined by 
a proper weighting of the contributions of scattering from a volume element 
of the medium, according to relative number density of the different particles 
and their respective phase functions. This more complicated method was not 
used in this exploratory study, but it would be advisable to use the addition- 
al precision attainable with the method for actual calibration of the satellite 
radiometer. 

The fluxes F^ a (- y) and Fg S a (- y) due to diffuse transmission of the 
incident radiation and backscattering of the surface -reflected radiation by 
the aerosol component of the atmosphere were computed by the matrix method and 
are shown at the bottom of figure 12. Comparison of the curves for F(j a (- y) 
with the equivalent results for a Rayleigh atmosphere, given in figure 3, shows 
that the flux transmitted diffusely by the aerosols is considerably greater 
than that transmitted diffusely by the Rayleigh atmosphere, particularly at 
the longer wavelengths. This difference is a result of the different types 
of scattering phase functions for the different types of particles. Aerosol 
particles scatter predominantly into the forward direction, while the 



17 







0.6 07 0.8 

Wavelength [jum] 



Figure 13. --Total relative flux of solar radiation downward at White 
Sands for various solar zenith angles. 

scattering pattern for Rayleigh particles is symmetric for the forward and 
the backward directions. This difference in scattering patterns is responsi- 
ble for the fact that the amount of surface-reflected radiation scattered 
back to the surface by the aerosol atmosphere is less than that scattered 
downward by the Rayleigh atmosphere (fig. h) . 

A reasonably good approximation to the total flux incident at the surface 
of White Sands can now be obtained by adding together the components discussed 
above. The total flux downward at the surface is 



F g (- y)=F D (- y) + |Fd(- ^ +F gs<- u)] R+ [fJ (- y) + F gs a (-y )] 



(8) 



where superscripts R and a indicate Rayleigh and aerosol components, 
respectively. Fjjv- \i ) is computed from (5), with aerosols, Rayleigh 
particles, and ozone taken into account. The one element neglected is multi- 
ple scattering between aerosol and Rayleigh particles. Curves of the total 
relative flux incident at the surface for various zenith angles%f the sun 
are shown in figure 13. The flux assumed incident at the top of the atmos- 
phere for each zenith angle is indicated by an arrow. It is probable that 
absorption by ozone is responsible for the major part of the dip of the curves 
in the vicinity of 0.6 y m, even though a number of parameters are combined 
in defining the curves. 



18. 

Because of lack of information on the bidirectional reflectance of the 
white sand surface, we assume that the intensity Ip;( + ^ )> resulting from re- 
flection of the flux Fg(-y ) incident on the surface is isotropic. Then we 
compute the radiation intensity from the nadir direction at the top of the 
atmosphere; this is the quantity that would be available for calibration of 
the satellite radiometer. As stated previously, this intensity consists of 
the following of components: 



K+u )=I gD (+y)+ [l gd (+v)+I s ( + ii) ] R+ [ I gd( + ^ J ) =I s( + i J )] 



(9) 



In the use of this equation, the total optical thickness of the turbid 
atmosphere is considered for the first term on the right and multiple 
scattering is accounted for by the quantities within each of the brackets. 
Gross terms between quantities within the brackets are neglected. 

The relative contributions of the various components to the total intensity 
at the top of the atmosphere can be seen for© = 0° in figure ll; and © = 60 
in figure l£. By far the major component in both cases, as well as in those 
not shown, is the intensity I p(+y ) resulting from direct upward transmission 
of the surface intensity I_(+y ). For this component, only an attenuation of 
IgC+y) is involved; in computing this, the attenuation by aerosols, Rayleigh 
particles, and ozone is taken into account, so relatively high precision 
values are obtained for this major component. Computations of the other 
components are less precise, but their smaller magnitudes tend to decrease 
their relative contribution to the overall errors of the results. 

We obtain the outward intensity in absolute energy units by multiplying the 
values of relative intensity at the top of the atmosphere (shown in fig. ill 
and 15>) by the quantity F Q / tt , where F Q is the actual flux at the top of the 
atmosphere. This is the quantity we have been seeking for calibration of the 
satellite radiometer. The absolute intensities emitted outward from the nadir 
direction at the top of a turbid atmosphere over White Sands are shown for 
several different solar zenith angles in figure 16. The values of F Q as a 
function of wavelength were taken from Thekaekara (1970) ; these values have 
been adopted by the National Aeronautics and Space Administration for engineer- 
ing design purposes. Note the spectral response of the radiometer to be 
calibrated in relation to the energy available. A typical transmission curve 
for the radiometer is shown in relative units in figure 16. 

3. REQUIRED MEASUREMENTS 

Valuable as model calculations are for putting the problem on a solid 
theoretical foundation, they are still no substitute for actual measurements. 
It is suggested that to provide a method for calibrating satellite radio- 
meters using the White Sands area as a "standard surface", three types of 
measurements are necessary: 

A. Measurements of the Direct Flux F D at the Surface 

Probably the largest source of error in the proposed calibration method is 
the lack of precise knowledge of all the atmospheric properties involved. 
As pointed out above, atmospheric particulates vary in type, amount, and 



19. 



a* 0.3 




0.0 



0.5 0.6 0.7 0.8 
Wavelength (/xm) 



0.9 



Figure lit. --Components of the relative intensity outward from the nadir direct- 
ion at the top of a turbid atmosphere over White Sands for a solar zenith 



angle 0=0° 
to o 



0.4 



c 
fl) 


0.3 



> 

o 

o 

ex. 


0.2 




0.1 




0.0 



T 




Total 
'.D 



a R " 
'gd + 'gd 



0.5 



0.6 



0.7 



0.8 



0.9 



+ I 



Wavelength [fjum] 

Figure 15. --Components of the relative intensity outward from the nadir 
direction at the top of a turbid atmosphere over White Sands for a solar 
zenith angle Go = 60° 



20. 



E 
1 



0.040 



0.035 - 



0.030 - 



0.025 



0020 



E 
u 

« 0015 

c 

0.010 
0.005 
0.000 




_k'_ 



Relative Filter 
Transmission 

/ 



1 



0.5 0.6 0.7 0.8 09 

Wavelength (/urn) 



1.0 



05 







Figure 16. --Total absolute intensity outward from the nadir direction at 
the top of a turbid atmosphere over White Sands, (incident flux as given 
by Thekaekara) . 

optical characteristics; the aerosol optical thickness generally is equal to 
or larger than that of the Rayleigh atmosphere in the 0.5 to 0.9 ym wavelength 
region, and the amount of ozone is variable with time and location. Measure- 
ments of the flux of the directly transmitted solar energy, Fjj(-p Q ), can be 
used to derive corrections for these unknown atmospheric effects. 

Figures 2, 3, and k show that the dominant source of energy incident on the 
surface at White Sands is the directly transmitted solar flux, Fjj(-y ) ; 
figure 8 shows that the dominant intensity component from the nadir direction 
at the top of the atmosphere is IgD( + ^ ), due to radiation which is trans- 
mitted directly upward through the atmosphere. To correct I„jj( + y ) for 
atmospheric effects at a given time, a determination of the atmospheric 
optical thickness t can be derived from measurements of Fjj(- V ) . This 
value of t can be used to correct the upward intensity I jj(+y) for atmos- 
pheric attenuation between the surface and the satellite. As an added bonus, 
the measurements of Fjj(- y ) will serve as a check on measurements of global 
radiation incident at the surface. 



The optical thickness of the atmosphere in the normal direction for the 
direct beam is simply 



t - 



V F n (- u W 



(10) 



D v ~^o 



21. 

where F Q (- y Q ) is known from solar constant determinations (Thekaekara, 1970), 
6° is known from astronomical formulae (Robinson, 1966), and Fj)(- y ) is 
measured. Then the intensity Igi)( + V ) in the nadir direction at the top of 
the atmosphere is 

IgD(+v)'- I g (+u)e~ T (11) 

B. Measurements of Global Flux 

If the total global energy flux F„(-y ) at the surface is known, the out- 
ward intensity I„(+y) can be determined for a Lambert surface by (6) 
written as 

I g (+y) = £ F g (-y) (12) 

TT 

If the surface is non-Lambert in character, then R/ ^ must be replaced by a 
bidirectional reflectance p (+y , - y , <f> ), but otherwise the relation 
holds. Since the diffuse and direct radiation fluxes are both affected by 
atmospheric conditions, and the diffuse flux is not a unique function of 
atmospheric optical thickness x , then it is necessary to measure F~(-y ) at 
the White Sands surface. 

C. Measurements of Surface Reflectance 

There are a number of aspects of surface reflectance to be considered. 
First, we wish to know the extent to which the surface at White Sands is a 
Lambert surface. There are not enough measurements at the present time to 
determine this completely. The relatively few measurements available 
(Goulson, et al. 1965, Chen, et al. 1967, Salomons on 1968) show that the 
white gypsum sand approximates a Lambert surface more closely than any other 
natural surface for which measurements are available. For instance, from 
laboratory measurements, part of which are reproduced in figure 17, Coulson 
et al. (1965) found that for illumination from the direction of the surface 
normal, the relative reflectance p varied only about 6 percent as the 
direction of view varied from = 5° to = 80°. However, for = 53°, 
p varied more than 25 percent for the same range of 9, and the curve Is not 
symmetric around the nadir direction. Furthermore, the value of p in the 
nadir direction (0=0°) increased from 66 percent for 0=0° to 75 percent for 
0=53°. Other measurements tend to corroborate these variations. Thus, one 
can conclude that total reflectance and bidirectional reflectance from the 
nadir direction are both relatively strong functions of angle of incidence 
for monodirectional radiation. This variation, which affects the determina- 
tion of Ig( + ^ ), must be determined if the T ihite Sands area Is to be used for 

reasonably accurate calibration of the satellite radiometer. On the other 
hand, reflection of the diffuse fluxes F DS (-y ) 3 Fj C-y ), F-q * ( - p ), and 
F,(-y ) should be nearly isotropic over the hemisphere, but of course the 
magnitude of the first three must be dependent on the total reflectance. 



An additional complication is introduced by the change of reflectance with 
wavelength. As was shown by figure 5, this is considerable in the sense 
that reflectance increases as wavelength increases from A = 0.5 to 0.75y m. 
It is fortunate, however, that according to available measurements, the 



22. 



110 



c 


100 


<D 




U 




L. 




0) 




O- 




<D 


90 


u 




c 




o 








u 




0) 






80 


<D 




CxL 




15 




c 







70 






u 




0) 




L. 




"D 




CO 


60 




50 




80 60 40 20 20 40 60 80 

Nadir Angle (Degrees) 



Figure 17. --Bidirectional reflectance of white gypsum sand as a function 
of nadir angle in the principal plane for two different zenith angles 
of the source at a wavelength of 0.61;3jjm (after Coulson, 1965). 

angular dependence of reflectance is nearly independent of wavelength; only 
the magnitude is wavelength dependent. This means that reflection data 
must be available at a number of wavelengths over the range of interest. It 
is not quite clear to what extend the curve of figure 5> will suffice to 
deterimine spectral dependence; it should be at least corroborated by in situ 
measurements. 

Another source of concern is the possibility that reflectance of the white 
gypsum sand will change in time. On a time scale of years or decades, it is 
likely that any significant changes of reflectance of the White Sands area 
would be due to changes of vegetation. The gypsum sand itself is constantly 
being renewed by surface deposition through evaporation of mineral-laden 
water percolating from below, and winds blow the surface material about, thus 
exposing uncontaminated reflecting material. Thus, long-term changes must be 
of minor significance for this study. This is not necessarily so in the case 
of short-term changes, however. For instance, we do not know how the reflec- 
tance changes with changes in the amount of moisture present on the surface, 
nor whether the material is bleached by long exposure to sunlight. If either 
of these effects, or others, cause short-period changes of reflectance, the 
amount of the change should be monitored so appropriate corrections can be 
made. It is considered likely that only the total reflectance would be 
affected, and that the wavelength variation and directional dependence would 
"emain unchanged, but this should be verified. 



23. 

Finally, the reflectance characteristics of the White Sands region should 
be determined for low spatial resolution. The field of view of present 
satellite radiometers encompasses an area several kilometers across. Such an 
area in the White Sands region includes sand dunes, valleys, scattered 
vegetation, and other features. Measurements should be made to establish the 
directional reflectance of such large areas from the nadir as a function of 
solar zenith angle and wavelength. This could be a one-time determination by 
aircraft -mounted instrumentation. Because the accuracy of the calibration 
method is directly dependent on the accuracy of this one set of measurements, 
every effort should be made to assure reliability and the highest possible 
accuracy for this determination. 

In summarizing the reflectance problem, we can state the following: 

(1) For a determination of the outward intensity component I-pC+y) a t the 
surface, both the direct flux Fj-)( _ y ) and bidirectional reflecrance 

p(-u ,c|> ;u,<f>) must be known accurately. It was suggested above that Fp(-y Q ) 
be measured; here we add a suggestion that p(+y,<{>;-u »<f> ) from the nadir 
direction be determined for all zenith angles of the sun at which calibrations 
are likely to be performed. 

(2) For a determination of the component of the intensity Ig.(+y) due to 
reflection of the diffuse incident light, the total reflectance R(y )must be 
known for all solar zenith angles at which calibrations are to be performed. 
By knowing R(u ), it will be possible, with the Lambert assumption for only 
this part, to compute this flux component as 

f" F (_u) _ F D (-y )]j (13) 

where F(-y) and F-nC-VU) are the flux components to be measured. The total 
outward intensity at the surface from the nadir direction becomes 

.C+y) = p(l;-y )F D C-y ) + [F(- y )-F D (-y )J £ (14) 

(3) The total reflectance of the white sand surface should be monitored 
continuously to detect possible short-period changes of reflectance. If such 
changes do occur, measurements should be made to characterize any correspond- 
ing changes of spectral or angular distribution of the reflected radiations. 

(M-) The directional reflectance of the White Sands area should be deter- 
mined as a function of wavelength and zenith angle of the sun. Spatial 
resolution should be similar to that of the satellite radiometer. Since the 
accuracy of the eventual calibration of the radiometer is directly dependent 
on the accuracy of this determination, the measurements should be of the 
highest possible accuracy and completeness. 

4. SUGGESTED INSTRUMENTATION AND OBSERVATIONS 

A. Instrumentation for Routine Operations 

It is suggested that the equipment for routine operations at a surface 
station at White Sands consist of two radiation instruments and a two-pen 
strip chart recorder. The two instruments should be single-filter devices, 



I, 



2U. 



one a filter pyrheliometer. The instrument designs might be similar to the 
f oil owing: 

Filter pyrheliometer - to measure the energy flux Fjj(-u ) in the direct 
solar beam in the spectral interval of interest. The optical thickness of 
the atmosphere will be determined from this quantity. The device would be 
oriented toward the sun by means of a clock-driven equatorial mount. The 
field of view would 'be as small as feasible (perhaps a 2° half -angle cone), 
to decrease the amount of circumsolar radiation received. It is likely that 
the large changes of -ambient temperature to which the instrument would be 
subjected and the high accuracy required will combine to demand complete 
temperature control of detector and associated electronics. Both instrument 
and mount must be adapted to a hostile environment. The instrument should be 
calibrated at least once every 6 months. 

Filter pyranometer - to measure the global flux F„(-ij ) incident on the 
surface from the upward hemisphere and the global flux F (+u ) reflected from 
the white sand surface. The first of these is the basic measurement from 
which the large-scale outward flux will be determined; the second is for 
detecting possible short-period changes of surface reflectance by the ratio 
F ( + u)/F (- u) . The most satisfactory receiver for both fluxes would seem 
to be an integrating sphere with the entrance aperture of the detector set 
into the side of it. The aperture would normally be in an upward-facing 
position to receive the downward radiation from sun and sky, yielding the 
flux F~(- y) . It would occasionally be rotated to a downward-facing position 
to receive F„( + \i) } the upward radiation reflected from the surface. The 
rotation would be accomplished by mounting the instrument on a horizontal 
axis, around which the assembly could be rotated back and forth through a 
l80° angle on a preset program (perhaps 5> minutes of every hour) by an 
electric motor. The instrument would be located on a flat expanse of typical 
white sand at a height of k to 6 feet above the surface. The entire installa- 
tion would have to be weather proof, the aperture of the sphere covered by an 
optical glass hemisphere, and the detector and electronics temperature 
controlled. Condensation of moisture on the glass hemisphere, always a 
problem in this device, would be minimized by providing temperature control 
or nitrogen flushing of the sphere. The system should be calibrated at least 
once every 6 months. 

Strip-chart recorder - the two watchwords for the recorder are stability 
and reliability of operation. If the response of the recorder is stable it 
can be calibrated to yield accurate measurement data. Its operation must be 
reliable to minimize maintenance requirements and to yield continuous traces 
of impressed signals over long periods of time. Probably the most frequent 
problems of recorder operation are the inking of pens and the movement of the 
recorder chart. The recorder should be located in an air conditioned enclos- 
ure at a position sufficiently close to the receiving instruments to allow 
short transmission lines. It should be calibrated at least once every 6 months 

B. Instrumentation for Special Measurements 

Aircraft-mounted radiometer - for determining the directional reflectance 
of the White Sands area, and the dependence of directional reflectance on 
zenith angle of the sun. If a new instrument were to be designed for this 



2*. 

purpose, it logically would be a single-filter instrument responding to the 
same wavelength range as that sensed by the satellite radiometer. It is 
likely, however, that existing instruments could be used for the purpose, 
either a duplicate of the satellite radiometer or the aircraft spectrometer 
of W. A. Hovis, NASA, Goddard. The choice would be that of the experimenter, 
but the measurements should be very accurate. One firm requirement is that 
they be made at a time when the surface instrumentation is in operation. 
They should be repeated at least once a year as a check on the entire calibra- 
tion system. 

C. Auxiliary Observations 

A source of concern in the calibration of the satellite radiometer is the 
possible existence of clouds over the White Sands area at the time of the 
calibration. It may be possible to tell either from the satellite pictures 
or from the records of the surface instruments whether clouds were present, 
but visual observations by a weather observer would be valuable as corrobo- 
rative evidence. Thus it is suggested that arrangements be made for weather 
observers of the U.S. Army weather station of the White Sands Missile Range 
to take special cloud observations at the times selected for calibration of 
the satellite radiometer. 

5. DISCUSSION 

In view of these considerations, the use of the White Sands area as a 
standard surface for the calibration of the visible channel of the satellite 
radiometer appears feasible, and the proposed method should permit derivation 
of a reasonably accurate absolute calibration of the instrument. In addition, 
the long-term stability of the surface at White Sands should yield calibrations 
essentially independent of time, from which trends of radiometer performance 
can be not only detected but corrected for quantitatively. 

Attractive as the use of the White Sands area appears for the purpose, some 
thought should be given to the use of other areas. In the first place, the 
upward intensity over White Sands wll be high and will thus give a high point 
on the calibration curve. Perhaps additional points are needed. It is 
likely that another calibration point, at a low intensity value, can be ob- 
tained by using space as a target. One or two well-defined points between 
these extremes would seem desirable. 

One method of obtaining different intensities would be to use the White 
Sands area at different zenith angles of the sun. This might be feasible for 
an earth- synchronous satellite, but not for a sun- synchronous spacecraft. 
However, the errors due to atmospheric effects at larger solar zenith angles 
make more attractive the selection of an area with lower reflectance. 

Various types of surfaces can be thought of as candidates. A water surface 
is known to have low reflectance at small zenith angles of the sun, and parts 
of the oceans are cloud-free a large percentage of the time. The main dis- 
advantage of the sea surface as a target is that the reflectance may change 
with wind speed and particle content. Also, it would be difficult to put a 
surface observation station on the ocean to obtain ground truth data. A 
second set of possibilities is snow fields or glaciers, but features such as 



26. 

variable reflectance, high -latitude locations, and large amounts of cloudiness 
over snow and ice fields decrease their attractiveness as calibration surfaces. 
A third possibility is some other land surface in subtropical desert areas, 
where the sun ranges over small zenith angles, cloudiness and precipitation 
are at a minimum, and an observation station can be established. This last 
approach seems to be the best of the three. 

For a subtropical desert station one immediately thinks of the Middle East 
--the Sahara and the Arabian Desert. Satellite photographs show wide 
expanses of more or less uniform reflectance throughout the region which 
could be identified by means of prominent landmarks. One is faced, however, 
with establishing a surface observation station which would be reliable and 
which could be operated continuously. Without surface observations it is 
likely that errors introduced by unknown atmospheric properties would make 
any derived calibrations of questionable value. The feasibility of establish- 
ing a surface station at an oasis should be investigated. 

One other site which looks attractive as a calibration surface is a dry lake 
bed on a high plateau of Bolivia. This area, the Salar de Uyuni, is a rough- 
ly circular salt flat about 60 miles diameter. It has an elevation of 12,000 
feet and is located at about latitude 20°S. It shows up well on satellite 
photographs and appears to be relatively uniform in brightness. Its high 
altitude, low latitude, small amount of cloudiness, and lack of large sources 
of pollution all Indicate that the area would be ideal for calibration 
purposes. Further investigation is needed as to the actual homogeneity of the 
surface and the feasibility of establishing a surface observation station at 
the site. 



27. 



6. REFERENCES 
Chandrasekhar , S. , Radiative Transfer , Oxford, Clarendon Press, 1950, 393 pp. 

Chen, H.S., Rao, C.R.N. , and Sekera, Z. , "Investigations of the Polarization 
of Light Reflected by Natural Surfaces," Scientific Report No. 2, Contr. 
AF 19(628)-3850, University of California, Los Angeles, 1967, 96 pp. 

Coulson, K.L., Dave, J.V., and Sekera, Z, Tables Related to Radiation Emer g- 
ing from a Planetary Atmosphere with Rayleigh Scattering , University of 
California Press, Berkeley, 1960, 548 pp. 

Coulson, K.L. , Bouricius , G.M., and Gray, E.L., "Effects of Surface Reflection 

on Radiation Emerging from the Top of a Planetary Atmosphere," T.I.S. 

Report R65SD64, Space Sciences Laboratory, General Electric Co., 
Philadelphia, 1965, 149 pp. 

Coulson, K.L., "Effect of Surface Reflection on the Angular and Spectral 
Distribution of Skylight," Journal of Atmospheric Sciences , Vol. 25, 
No. 5, 1968, PP- 759-770. 

Elterman, L. , "UV, Visible and IR Attenuation for Altitudes to 50 km, 1968," 
Environmental Research Papers No. 285, Air Force Cambridge Research 
Laboratories, Bedford, Mass., 1968, 49 pp. 

Hovis, W.A. , Private communication,' 1971. 

Junge, C. , "Gesetzmassigkeiten in der Grossenverteilung atmospharischer 

Aerosol uber dem Kontinent, Berickte Deutsche Wetterdienst U.S. -Zone , 
No. 35, 1952, pp. 261-277. 

Robinson, N. , ed. Solar Radiation , Elsevier Publishing Co. New York, 1966, 
347 pp. 

Salomonson, V.V. , "Anistropy in Reflected Solar Radiation," Atmospheric 

Science Paper No. 128, Colorado State University, Fort Collins, 1968, 
143 pp. 

Thekaekara, M.P. , "Proposed Standard Values of the Solar Constant and the 
Solar Spectrum," Journal of Environmental Sciences Vol 13, No. 4, 
1970, pp. 6-9. 

Twomey, S. , Jacobowitz, H. , and Howell, H.B., "Matrix Method for Multiple- 
Scattering Problems," Journal of Atmospheric Sciences , Vol. 23, No. 3, 
1966, pp. 289-296. 



* U. S. GOVERNMENT PRINTING OFFICE : 1972 — 51 1. 3 9 (71,) 



(Continued from inside front cover) 

NESS 57. Table of Scattering Function of Infrared Radiation for Water Clouds, Giichi 
Yamamoto, Masayuki Tanaka, and Shoji Asano, April 1971. (COM-71-50312) 

NESS 58. The Airborne ITPR Brassboard Experiment, W. L. Smith, D.T. Hilleary, E. C. 
Baldwin, W. Jacob, H. Jacobowitz, G. Nelson, S. Soules, D. 0. Wark, March 
1972. 

NESS 59. Temperature Sounding From Satellites, S. Fritz, D. Q. Wark, H. E. 
Fleming, W. L. Smith, H. Jacobowitz, D. T. Hilleary, and J. C. 
Alishouse, July 1972. 

NESS 60. Satellite Measurements of Aerosol Backscattered Radiation From the 
Nimbus F Earth Radiation Budget Experiment, H. Jacobowitz, W. L. 
Smith, and A. J. Drummond, August 1972. 

NESS 61. The Measurement of Atmospheric Transmittance From Sun and Sky With an 

Infrared Vertical Sounder, W. L. Smith and H. B. Howell, September 1972. 



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